Shawne Merriman is furious his good pal and former teammate Antonio Gates didn't make the cut for the Hall of Fame on Thursday ... and now, he's demanding an explanation from the committee.
Lights Out didn't hold back when TMZ Sports spoke with him about the Chargers legend's HOF snub ... saying point blank, "This is pretty much an embarrassment, man."
"This one needs an explanation," he said. "Because if you watch football, in the last two decades, there's no way in the world that Antonio Gates is not a first-ballot Hall of Famer."
Instead of Gates, the committee chose to put in Dwight Freeney, Devin Hester, Andre Johnson, Julius Peppers and Patrick Willis.
Clearly, Merriman thinks Gates got robbed ... adding, "I was shocked just like every other football fan was."
The sad part ... Merriman -- who played with Gates from 2005 through 2010 -- said he knows Gates is feeling down about it all.
"I know this has to bother him some," Merriman said.
Of course, Shawne is confident Gates will get his gold jacket sooner rather than later ... but in the meantime, he really wants to know why it didn't happen this week.
"I know they don't come out and give too many explanations," he said, "but this one needs an explanation."